Edge lighted device with polarization

ABSTRACT

A planar light source device includes a primary light source ( 1 ), a light guide ( 3 ) which leads the light emitted from the primary light source ( 1 ), and having a light incident face ( 31 ) to which the light emitted from the primary light source ( 1 ) comes in, and a light emitting face from which the led light goes out, a light deflection element ( 4 ) arranged adjacent to the light emitting face ( 33 ) of the light guide ( 3 ), and a polarization separation element ( 6 ) disposed on the side of a light outgoing surface ( 42 ) of the light deflection element ( 4 ), and having a function of transmitting one of polarization components of the incident light and reflecting the other of the polarization components. The full width at half maximum of the luminance distribution of the incident light upon the polarization separation element ( 6 ) in an XY plane parallel to the travel direction of the light in the light guide ( 3 ) is 25° or less. The reference symbol  7  indicates a liquid crystal display element.

TECHNICAL FIELD

The present invention relates to an edge light system planar lightsource device constituting a liquid crystal display device or the likefor use as a display unit in a notebook-size personal computer, liquidcrystal television, cellular phone, portable information terminal(personal digital assistants [PDA]) or the like, particularly toimprovement of a planar light source device using a polarizationseparation element.

BACKGROUND ART

In recent years, color liquid crystal display devices have been broadlyused as monitors for portable notebook-size personal computers, personalcomputers and the like, or as display units for liquid crystaltelevision sets, video integrated liquid crystal television sets,cellular phones, portable information terminals and the like in variousfields. With an increase of an information processing amount,diversification of needs, adaptation to multimedia and the like,enlargement of a screen, and enhancement of definition of the liquidcrystal display device have been vigorously advanced.

The liquid crystal display device basically comprises a backlight unitand a liquid crystal display element unit. As the backlight unit, thereis an under light system in which a light source is disposed right underthe liquid crystal display element unit or an edge light system in whicha light source is disposed facing a side face of a light guide. From aviewpoint of a compact liquid crystal display device, the edge lightsystem has been frequently used.

Additionally, since a polarization direction of light emitted from thebacklight unit has heretofore been uneven, about half of the light hasSeen absorbed and wasted by a polarization plate disposed on anincidence side of the liquid crystal display unit.

As a solution of this problem, as described in JP(A)-9-506984, a methodis used in which a polarization separation element having a function oftransmitting one polarization component of the light and reflecting theother polarization component is disposed on an emitting face of thebacklight unit. A polarization transmission direction of thepolarization plate disposed on the incidence side of the liquid crystaldisplay element is matched with that of the polarization separationelement. Accordingly, the polarization component which has heretoforebeen absorbed by the polarization plate and wasted is reflected by thepolarization separation element, and returned to the backlight unit. Thelight returned to the backlight unit is reflected by the back surface ofthe light guide or the like, and strikes on the polarization separationelement again. The light returned to the backlight unit changes itspolarized state while reflection is repeated in the light guide. A partof the light which has entered the polarization separation elementpasses through the polarization separation element, and the other lightis reflected again. In this manner, if there is not any loss of quantityof light while the light reciprocates between the polarizationseparation element and the backlight unit, all the light eventuallypasses through the polarization separation element. There list not beany light that has heretofore been absorbed by the polarization plate ofthe liquid crystal display element unit.

However, in actuality, since there occurs a loss of quantity of lightevery time the light reflected by the polarization separation element isreflected by the backlight unit, luminance is not enhanced according tothe theory. Then, in order to enhance a ratio at which the lightreflected by the backlight unit strikes on the polarization separationelement again, it has been proposed in JP(A)-11-352479 that a regularreflection sheet be used on the back surface of the light guide. Tofurther enlarge a change of the polarized state by the reflection in thebacklight unit, it has been described in JP(A)-11-142849 that elongatedprisms be disposed on the back surface of the light guide, and formed insuch a manner as to extend in an oblique direction with respect to apolarization transmission plane of the polarization separation element.

However, even if these methods are used, the loss by the reflectionbetween the polarization separation element and the backlight unitcannot be sufficiently reduced, and an effect of enhancing the luminanceby the use of the polarization separation element is not sufficient.

DISCLOSURE OF THE INVENTION

Therefore, an object of the present invention is to provide a planarlight source device which enhances a polarization separation ability ofa polarization separation element and whose luminance is remarkablyhigh.

In view of the above-described problems of the conventional techniques,the present inventors have found that light entering a polarizationseparation element is concentrated on a normal-line direction of thepolarization separation element, so that transmittance with respect topolarized light in a transmission axis direction of the polarizationseparation element can be enhanced, and a polarization separationability of the polarization separation element can be enhanced, and thusthe present inventors have reached the present invention.

That is, according to the present invention, there is provided a planarlight source device comprising: a primary light source; a light guideleading light emitted from the primary light source, and having a lightincident face to which the light emitted from the primary light sourcecanes in, and a light emitting face from which the led light goes out; alight deflection element disposed adjacent to the light emitting face ofthe light guide; and a polarization separation element disposed on theside of a light outgoing surface of the light deflection element, andhaving a function of transmitting one of polarization components of anincident light and reflecting the other of the polarization components,wherein a full width at half maximum of a luminance distribution of theincident light upon the polarization separation element in a directionparallel to a travel direction of the light in the light guide is 25° orless.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view showing a planar light sourcedevice according to the present invention;

FIG. 2 is an explanatory view of a shape of an elongated prism of alight incoming surface of a light deflection element of the planar lightsource device according to the present invention;

FIG. 3 is an explanatory view of the shape of the elongated prism of thelight incoming surface of the light deflection element of the planarlight source device according to the present invention;

FIG. 4 is an explanatory view of the shape of the elongated prism of thelight incoming surface of the light deflection element of the planarlight source device according to the present invention;

FIG. 5 is an explanatory view of the shape of the elongated prism of thelight incoming surface of the light deflection element of the planarlight source device according to the present invention;

FIG. 6 is an explanatory view of the shape of the elongated prism of thelight in coming surface of the light deflection element of the planarlight source device according to the present invention;

FIG. 7 is a schematic perspective view showing a light source deviceaccording to the present invention; and

FIG. 8 is an explanatory view of a full width at half maximum of anemitted light luminous intensity distribution (in XZ plane) of a lightdiffusion element of the planar light source device according to thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described hereinafter withreference to the drawings.

FIG. 1 is a schematic perspective view showing one embodiment of aplanar light source device according to the present invention. As shownin FIG. 1, the planar light source device of the present inventioncomprises: a light guide 3 whose at least one side end face is a lightincident face 31 and whose one surface substantially crossing the lightincident face at right angles is a light emitting face 33; a primarylight source 1 disposed facing the light incident face 31 of the lightguide 3 and covered with a light source reflector 2; a light deflectionelement 4 disposed on the light emitting face 33 of the light guide 3,and having a light incoming surface 41 and a light outgoing surface 42;a polarization separation element 6 disposed on the light outgoingsurface 42 of the light deflection element 4; and a light reflectionelement 5 disposed facing a back surface 34 of the light guide 3 on theopposite side of the light emitting face 33. FIG. 1 also shows a liquidcrystal display element 7 disposed on the polarization separationelement 6 of the planar light source device. These components constitutea liquid crystal display device.

The polarization separation element 6 transmits one polarizationcomponent (transmission polarization plane) of light from the lightdeflection element 4, emits the light upwards in FIG. 1, and reflectsthe other polarization component (reflection polarization plane) towardthe light deflection element 4. The element is disposed in such a mannerthat the transmission polarization plane agrees with a transmissiondirection of an incidence-side polarization plate of the liquid crystaldisplay element 7.

As the polarization separation element 6, an element is preferably usedin which a plurality of sheets each having double refraction propertiesand a predetermined thickness are stacked, and directionality of eachstacked sheet is set in such a manner that a refractive index differencebetween the adjacent sheets is large in the transmission polarizationplane, and reduced in the reflection polarization plane. As thepolarization separation element 6, a combination of a film obtained bystacking cholesteric liquid crystal layers with a ¼ wavelength plate isalso preferable. In this case, circularly polarized light in a certaindirection is transmitted, and circularly polarized light in a reversedirection is reflected in the cholesteric liquid crystal layers. At thistime, linearly polarized light can be taken out, when the ¼ wavelengthplate is disposed on the cholesteric liquid crystal layers. Thepolarization separation element is disposed in such a manner that thedirection of the linearly polarized light is regarded as thetransmission polarization plane, and the direction agrees with thetransmission direction of the incidence-side polarization plate of theliquid crystal display element 7.

To enhance the polarization separation ability of the polarizationseparation element 6, it is necessary to concentrate a luminancedistribution of the light entering the polarization separation element 6in the normal-line direction. The direction of the incident light ispreferably set to 25° or less with respect to the normal-line directionof the polarization separation element 6, because transmittance withrespect to the transmission polarization plane of the polarizationseparation element 6 and reflectance with respect to the reflectionpolarization plane of the polarization separation element 6 areenhanced. The direction of the incident light with respect to thenormal-line direction of the polarization separation element 6 is morepreferably 20° or less, further preferably 15° or less.

Therefore, a full width at half maximum of the luminance distribution ofthe incident light upon the polarization separation element 6 needs tobe set to 25° or less in an XZ plane (direction parallel to a directionin which the light travels in the light guide) including both adirection vertical to the light incident face 31 of the light guide 3and the normal-line direction of the light emitting face 33. The fullwidth at half maximum is preferably 20° or less, further preferably 15°or less. On the other hand, when the full width at half maxima is set tobe excessively small, a view field angle of the liquid crystal displaydevice is excessively small. Therefore, the full width at half maximumof the luminance distribution in the XZ plane of the incident light uponthe polarization separation element 6 is set to preferably 5° or more,more preferably 7° or more, especially preferably 10° or more. In thepresent invention, the full width at half maximum of the luminancedistribution in the XZ plane of the light emitted from the lightdeflection element 4 may be set in the above-described range in order toset the full width at half maximum of the luminance distribution of theincident light upon the polarization separation element 6 as describedabove. It is to be noted that the full width at half maximum of theluminance distribution in the present invention refers to an angle offull width of a spread angle in a half value with respect to a peakvalue in the luminance distribution.

In general, as shown in FIG. 1, in the planar light source device inwhich as the light deflection element 4 is used an element formed byarranging a large number of elongated prisms extending in a directionsubstantially parallel to the light incident face 31 of the light guide3 on one surface, and the light deflection element 4 is disposed in sucha manner that the elongated prism forming surface faces the light guide,the luminance distribution of the light going out of the lightdeflection element 4 differs with the direction. Usually, the luminancedistribution of the outgoing light from the light guide 3 in the XZplane is narrowed, and further the luminance distribution can be furthernarrowed by a elongated prism shape or the like of the light deflectionelement 4. However, in a YZ plane (plane parallel to the light incidentface 31 of the light guide 3, i.e., direction vertical to a direction inwhich the light travels in the light guide) including both the directionparallel to the elongated prism and the normal-line direction of thelight emitting face 33, the luminance distribution of the outgoing lightfrom the light guide 3 is also broad, and it is difficult tosufficiently narrow the luminance distribution with use of the lightdeflection element 4. Therefore, the full width at half maximum of theluminance distribution of the incident light upon the polarizationseparation element 6 in the YZ plane is set to preferably 50° or less,more preferably 45° or less, especially preferably 42° or less. On theother hand, when the full width at half maximum is set to be excessivelysmall, the view field angle is excessively narrow in the liquid crystaldisplay device. Therefore, the full width at half maximum is set topreferably 5° or more, more preferably 70 or more, especially preferably10° or more. In the present invention, the full width at half maximum ofthe luminance distribution in the YZ plane of the outgoing light fromthe light deflection element 4 may be set in the above-described rangein order to set the full width at half maximum in the luminancedistribution of the incident light upon the polarization separationelement 6 as described above.

In the present invention, an average value of the full width at halfmaximum in the luminance distribution of the incident light upon thepolarization separation element 6 in the XZ plane and the YZ plane ispreferably 33° or less, more preferably 30° or less, further preferably28° or less, especially preferably 25° or less. The full width at halfmaximum of the luminance distribution in the XZ and YZ planes of theoutgoing light from the light deflection element 4 may be set in theabove-described range in order to set the full width at half maximum inthe luminance distribution of the incident light upon the polarizationseparation element 6 as described above.

Moreover, as shown in FIG. 7, in a planar light source device in which aspot-like light source is used as the primary light source 1, an elementhaving a large number of arc elongated prisms juxtaposed on one surfaceso as to surround the spot-like primary light source 1 is used as thelight deflection element 4, and the light deflection element 4 isdisposed in such a manner as to dispose the elongated prism forgingsurface on the light guide side, the luminance distribution of the lightgoing out of the light deflection element 4 can be narrowed both in thedirection parallel to the direction in which the light travels in thelight guide and the direction vertical to the light traveling directionin the light guide. Therefore, the planar light source device is verysuitable for enhancing the polarization separation ability of thepolarization separation element 6. In this case, with regard to the fullwidth at half maximum of the luminance distribution of the incidentlight upon the polarization separation element 6, and that of theluminance distribution of the outgoing light from the light deflectionelement 4, an average value of the full width at half maximum in adirection parallel to a direction in which the light enters the lightguide, and that in a direction vertical to the direction in which thelight travels in the light guide is set to preferably 15° or less, morepreferably 12° or less, further preferably 10° or less.

Moreover, the full width at half maximum in the luminance distributionof the incident light upon the polarization separation element 6, andthat in the luminance distribution of the outgoing light from the lightdeflection element 4 need to be set to 25° or less, preferably 20° orless, further preferably 15° or less in the direction parallel to thedirection in which the light travels in the light guide. On the otherhand, when the full width at half maximum is set to be excessivelysmall, the view field angle of the liquid crystal display device isexcessively small. Therefore, the full width at half maximum is set topreferably 5° or more, more preferably 7° or more, especially preferably10° or more.

Furthermore, in the direction vertical to the direction in which thelight travels in the light guide, the full width at half maximum in theluminance distribution of the incident light upon the polarizationseparation element 6, and that in the luminance distribution of theoutgoing light from the light deflection element 4 are preferably 30° orless, further preferably 25° or less, especially preferably 20° or less.On the other hand, when the full width at half maximum is set to beexcessively smell, the view field angle of the liquid crystal displaydevice is excessively snail. Therefore, the full width at half maximumis set to preferably 5° or more, more preferably 7° or more, especiallypreferably 10° or more.

FIGS. 2 to 6 are explanatory views of a shape of an XZ section of theelongated prism of the light deflection element 4 for use in the planarlight source device of the present invention. In the light deflectionelement 4, one of major surfaces is regarded as the light incomingsurface 41, and the other major surface is regarded as the lightoutgoing surface 42. A large number of elongated prisms aresubstantially juxtaposed/arranged on the light incoming surface 41. Eachelongated prism comprises two prism faces of a first prism face 44positioned on the side of a light source, and a second prism face 45positioned on the side distant from the light source. In the embodimentshown in FIG. 2, both the first prism face 44 and the second prism face45 are flat faces. In the planar light source device of the presentinvention, an apex angle (α+β) of the elongated prism is in a range ofpreferably 35° to 80°, more preferably 35° to 75°, further preferably40° to 70°. In this range, a direction (direction of peak light) inwhich intensity of the light going out of the light deflection element 4is highest can be substantially matched with the normal-line directionof the polarization separation element 6, and a high polarizationseparation ability in the polarization separation element 6 is obtained.

In a vertical direction (XZ plane) vertical to an incident face of thelight guide 3, to further concentrate the luminance distribution of theoutgoing light from the light deflection element 4 in the normal-minedirection of the polarization separation element 6, as shown in FIGS. 3to 6, an inclination angle of the second prism face 45 of the lightdeflection element 4 is preferably formed in such a manner as toincrease as approaching to the light outgoing surface 42. As a result,the outgoing light totally reflected by the whole second prism face canbe condensed in a certain direction, and it is possible to emit lighthaving high directivity and large luminance of peak light. In this case,the second prism face 45 nay be formed of a convex curved face, or twoor more flat faces and/or convex curved faces having differentinclination angles.

When the second prism face 45 is formed of the convex curved face asshown in FIG. 3, a ratio (r/P) of a curvature radius (r) of the convexsurface to a pitch (P) of the elongated prism is in a range ofpreferably 2 to 50, more preferably 5 to 30, further preferably 7 to 20,especially preferably 7.5 to 15 in order to further enhance a condensingproperty by the light deflection element 4. A ratio (d/P) of a distance(d) between a virtual plane Q connecting a prism apex portion of thesecond prism face 45 to a prism bottom portion thereof, and the convexcurved face with respect to the pitch (P) of the elongated prism is in arange of preferably 0.05 to 5%, more preferably 0.1 to 3%, furtherpreferably 0.2 to 2%, especially preferably 0.7 to 1.5%.

Moreover, when the second prism face 45 is formed of two or more flatfaces and/or convex curved faces having different inclination angles,the number of the flat faces or the convex curved faces is set topreferably three or more, more preferably five or more, furtherpreferably six or more. When the face number is excessively small, thecondensing property by the light deflection element 4 drops, and aluminance enhancing effect tends to be impaired. On the other hand, whenthe face number is increased, the direction of the peak light over thewhole second prism face 45 can be finely adjusted. Therefore, a degreeof concentration as a whole can be enhanced. However, the flat faceshaving different inclination angles have to be formed finely. Therefore,designing or manufacturing of a cutting tool for cutting a mold to forma prism pattern of the light deflection element 4 becomes complicated,and it is difficult to stably obtain the light deflection element 4having a certain optical characteristic. Therefore, the number of facesto be formed on the second prism face 45 is set to preferably 20 orless, more preferably 12 or less. The second prism face 45 is preferablyequally divided into flat faces and/or convex ailed faces, but does nothave to be necessarily divided in such a manner, and can be adjusted inaccordance with a desired outgoing light luminance distribution (in theXZ plane). A width (length of each face portion in a elongated prismsection) of each face having different inclination angle is set to arange of 4 to 47%, more preferably 6 to 30%, further preferably 7 to 20%with respect to the pitch of the elongated prism.

The second prism face 45 comprises at least two flat faces havingdifferent inclination angles, the inclination angles of these flat facesincrease closer to the light outgoing surface 42, and a difference ofthe inclination angle between the flat face closest to light outgoingsurface and that most distant from the light outgoing surface is set to15° or less. Consequently, a remarkably high light condensing effect canbe exerted, and a remarkably high luminance of the light source devicecan be obtained. The difference of the inclination angle between theflat face closest to light outgoing surface and that most distant fromthe light outgoing surface is in a range of preferably 0.5 to 10°, morepreferably 1 to 7°. It is to be noted that when three or more flat faceshaving different inclination angles are forced, the difference of theinclination angle is preferably set to the above-described range, but isnot especially limited to this range, When the second prism face 45 isstructured in this manner, a light deflection element having a desiredlight condensing property can be easily designed, and a light deflectionelement having the certain optical characteristic can be stablymanufactured. It is to be noted that a matter concerning the differenceof the inclination angle similarly applies even to a case where thesecond prism face comprises a plurality of convex curved faces, or acombination of the convex curved face with the flat face.

In the present invention, the inclination angle in the flat face or theconvex curved face refers to the inclination angle with respect to anelongated prism formed flat plane 43, that in the convex cured facerefers to an averaged inclination angle of tangent lines in a pluralityof positions of the convex curved face with respect to the elongatedprism formed flat plane 43, and an inclination angle of a straight line(chord) connecting opposite ends of the convex curved face say be used.

In the present invention, for example, as shown in FIGS. 4 and 5, atleast one of the flat faces having the different inclination angles maybe replaced with a convex curved face 52, 53, or 54 (49 to 51 are flatfaces), or all the flat faces may be replaced with convex curved faces.As shown in FIG. 6, all faces may be flat faces 46 to 48.

As to the shape of the convex curved face, the shape of the XZ sectionmay be set to be circular arc or non-circular arc. Furthermore, when aplurality of convex curved faces constitute the prism face, the shapesof the respective convex curved faces are preferably different, theconvex curved face having a circular arc sectional shape may be combinedwith that having a non-circular arc sectional shape, and at least oneconvex curved face is preferably formed into a non-circular arcsectional shape. When the plurality of convex curved faces are formedinto the circular arc sectional shapes, curvature of each convex curvedface may be changed. Examples of the non-circular arc sectional shapeinclude a part of an elliptic shape, a part of a parabolic shape and thelike.

Furthermore, the ratio (r/P) of the curvature radius. (r) of the convexcurved face to the pitch (P) of the elongated prism is set to a range ofpreferably 2 to 50, more preferably 5 to 30, further preferably 7 to 20,especially preferably 7.5 to 15. When this r/P is less than 2, orexceeds 50, a sufficient light condensing characteristic cannot beexerted, and the luminance tends to drop.

Moreover, when the second prism face 45 comprises a plurality of flatfaces or convex curved faces having different inclination angles, theratio (d/P) of the maximum distance d between the virtual flat plane Qconnecting the apex portion of the elongated prism to the bottom portionthereof, and a plurality of flat faces or convex curved faces (facesforming an actual prism face) with respect to the pitch (P) of theelongated prism is set to preferably 0.05 to 5% in order to secure thesufficient light condensing characteristic. This is because the lightcondensing characteristic tends to drop, and sufficient luminanceenhancement tends to be impossible, when d/P is less than 0.05% orexceeds 5%. The d/P is in a range of more preferably 0.1 to 3%, furtherpreferably 0.2 to 2%, especially preferably 0.7 to 1.5%.

Furthermore, in order to enhance the light condensing characteristic, asshown in FIGS. 4 and 6, at least two flat faces and/or convex curvedfaces are preferably formed in a region having a height h from the prismapex portion, and assuming that a height of the elongated prism is H,h/H is preferably 60% or less.

In the present invention, right/left distributing angles (inclinationangles of two prism laces with respect to a normal line) α, β of theprism apex angle with respect to the normal line may be equal ordifferent. However, to efficiently enhance the luminance in thesubstantial normal-line direction (referring to a range of ±10° in theXZ plane in a case where the normal-line direction is set to 0°), theangles are preferably set to different angles. In this case, thedistributing angle α positioned on the light source side is preferablyset to 40° or less, and β is set to a range of 25 to 50°. When there isa slight difference between the distributing angles α, β of the apexangle, a light use efficiency rises, and luminance can be furtherenhanced. Therefore, the distributing angle α is preferably set to arange of 25 to 40°, and the distributing angle β is preferably set to arange of 25 to 45°. An absolute value (|α−β|) of the difference betweenthe distributing angles α, β is set to a range of preferably 0.5 to 10°,more preferably 1 to 10°, further preferably 1 to 8°. It is to be notedthat in a case where the peak light in the outgoing light luminancedistribution (in the XZ plane) is in a direction other than thesubstantial normal-line direction, the distributing angles α, β of theprism apex angle are adjusted, so that the outgoing light luminancedistribution (in the XZ plane) having the peak light in a desireddirection can be obtained.

Moreover, when the distributing angle α is set to 20° or less, the lightuse efficiency can be raised, and the luminance can be further enhanced.When the distributing angle α is reduced, the light use efficiency canbe raised. However when the distributing angle α is set to beexcessively small, the apex angle of the elongated prism decreases, andit is difficult to form a prism pattern. Therefore, the distributingangle α is set to a range of preferably 3 to 15°, more preferably 5 to10°. In this case, to set the peak light in the outgoing light luminancedistribution (in the XZ plane) to a range of ±2° with respect to thenormal-line direction and to enhance normal-line luminance, thedistributing angle β may be set to a range of 35 to 40°.

When the distributing angle α is set to 20° or less in this manner, aratio (ratio L2/L1 of a length L2 of a straight line distant from thelight source with respect to a length L1 of a straight line close to thelight source) of lengths of two straight lines connecting the prism apexportion to a valley portion in the sectional shape of the elongatedprism is preferably set to 1.1 or more. This is because the lightincident on the prism face in the vicinity of the light source can beefficiently received by the prism face distant from the light source,the light use efficiency can be raised, and the luminance can be furtherenhanced, when L2/L1 is set to 1.1 or more. L2/L1 is more preferably1.15 or more, further preferably 1.17 or more. On the other hand, whenthe pitch P of the elongated prism is constant, and when the L2/L1 isset to be excessively large, the apex angle of the elongated prism tendsto decrease, and it is difficult to form the prism pattern. Therefore,L2/L1 is set to preferably 1.3 or less, more preferably 1.25 or less,further preferably 1.2 or less. A ratio (L2/P) of the length L2 of thestraight line distant from the light source with respect to the pitch Pof the elongated prism is preferably set to 1.25 or more for similarreasons. The L2/P is further preferably 1.3 or more, further preferably1.4 or more. On the other hand, when L2/P is set to be excessivelylarge, the apex angle of the elongated prism tends to decrease, and itis difficult to form the prism pattern. Therefore, L2/P is set topreferably 1.8 or less, more preferably 1.6 or less, further preferably1.5 or less.

In the present invention, the second prism face 45 which is a prism facedistant from the primary light source 1 as described above is formedinto a non-single flat face (referring to a face other than a singleflat face), and accordingly a distribution width can be sufficientlyreduced in the outgoing light luminance distribution (in the XZ plane)of the light going out of the light deflection element 4 in a case wherethe primary light source is disposed on the end face 31 of the lightguide 3. The prism face (first prism face 44) closer to the primarylight source 1 is more preferably formed into a similar shape, in a casewhere a ratio at which the light propagating in the light guide 3 isreflected by an end face 32 on the opposite side of the light incidentface 31 and returns is comparatively high, or the primary light sources1 are disposed on two facing end faces of the light guide 3. On theother hand, at a comparatively low ratio at which the light propagatingin the light guide 3 is reflected by the end face 32 on the oppositeside of the light incident face 31 and returns, the prism face in thevicinity of the primary light source 1 is preferably formed into asubstantially flat face. In the light deflection element 4 of thepresent invention, the vicinity of the apex portion of the prism facepreferably comprises a substantially flat face. Accordingly, it ispossible to more precisely form the shape of a transfer face of aforming mold member for forming the elongated prism, and a stickingphenomenon in disposing the light deflection element 4 on the lightguide 3 can be inhibited from being caused.

The light guide 3 is disposed in parallel with an XY plane, and forms arectangular plate shape as a while. The light guide 3 has four side endfaces, and at least one side end face of a pair of side end facessubstantially parallel to the YZ plane is assumed as the light incidentface 31. The light incident face 31 is disposed facing the primary lightsource 1, and the light emitted from the primary light source 1 entersthe light guide 3 from the light incident face 31. In the presentinvention, for example, the primary light source may be disposed onanother side end face such as a side end face 32 facing the lightincident face 31.

Two major faces substantially crossing the light incident face 31 of thelight guide 3 at right angles are positioned substantially in parallelwith the XY plane, and either one face (upper surface in the drawing) isthe light emitting face 33. A directive light emitting functionstructure formed of a rough surface, or a directive light emittingfunction structure formed of a lens face on which a large number ofelongated lenses such as elongated prisms, elongated lenticular lenses,and V-shaped grooves are formed substantially in parallel with the lightincident face 31 is disposed on at least one of the light emitting face33 and the back surface 34 on the opposite side thereof. Accordingly,while the light that has struck on the light incident face 31 is guidedin the light guide 3, light having directivity in an emitted lightdistribution in a plane (XZ plane) crossing both the light incident face31 and the light emitting face 33 at right angles is emitted from thelight emitting face 33. Assuming that an angle formed by the directionof peak of the emitted light distribution in the XZ plane with the lightemitting face 33 is a, the angle a is preferably set to 10 to 40°, andthe full width at half maximum of the emitted light distribution ispreferably set to 10 to 40°.

In the rough surface or the elongated lens formed on the surface of thelight guide 3, an average inclination angle θa by ISO4287/1-1984 ispreferably set to a range of 0.5 to 15° in order to improve a uniformityof luminance in the light emitting face 33. The average inclinationangle θa is in a range of further preferably 1 to 12°, more preferably1.5 to 11°. An optimum range of the average inclination angle θa ispreferably set in accordance with a ratio (L/t) of a length (L) of thelight guide 3 in a direction in which the incident light propagates withrespect to a thickness (t) of the light guide 3. That is, when the lightguide 3 having L/t of about 20 to 200 is used, the average inclinationangle θa is set to a range of preferably 0.5 to 7.5°, further preferably1 to 5°, more preferably 1.5 to 4°. When the light guide 3 having L/t ofabout 20 or less is used, the average inclination angle θa is set to arange of preferably 7 to 12°, further preferably 8 to 11°.

According to ISO4287/1-1984, a rough surface shape is measured using aprobe system surface roughness gauge, coordinate in a measurementdirection is assumed as x, and the average inclination angle θa of therough surface formed on the light guide 3 can be obtained from anobtained inclination function f(x) using the following equations (1) and(2). Here, L denotes a measurement length, and Δa is tangent of theaverage inclination angle θa:Δa=(1/L)∫₀ ^(L)|(d/dx)f(x)|dx  (1); andθa=tan⁻¹(Δa)  (2).

Furthermore, light emission ratio of the light guide 3 is in a range ofpreferably 0.5 to 5%, are preferably 1 to 3%. When the light emissionratio is smaller than 0.5%, a quantity of light emitted from the lightguide 3 is reduced, and there is a tendency that sufficient luminance isnot obtained. When the light emission ratio is larger than 5%, a largequantity of light is emitted in the vicinity of the primary light source1, attenuation of the light in an X-direction in the light emitting face33 is remarkable, and there is a tendency that the uniformity ofluminance in the light emitting face 33 drops. When the light emissionratio of the light guide 3 is set to 0.5 to 5% in this manner, the lighthaving a high-directivity emission characteristic can be emitted fromthe light guide 3 in such a manner that an angle (peak angle) of thepeak light in the emitted light distribution (in the XZ plane) of thelight going out of the light emitting face is in a range of 50 to 80°with respect to the normal line of the light emitting face, and the fullwidth at half maximum of the emitted light distribution (in the XZplane) is 10 to 40°. Emission direction of the light emitted can beefficiently deflected by the light deflection element 4, and there canbe provided a planar light source device having a high luminance.

In the present invention, the light emission ratio from the light guide3 is defined as follows. Light intensity (I₀) of the emitted light in anedge of the light emitting face 33 on the side of the light incidentface 31 and emitted light intensity (I) in a position at a distance Lfrom the edge on the light incident face 31 side satisfies a relation ofthe following equation (3), assuming that the thickness (Z-directiondimension) of the light guide 3 is t:I=I ₀·α(1−α)^(L/t)tm (3),where constant α is a light emission ratio, i.e. a ratio (%) at whichthe light goes out of the light guide 3 per unit length (lengthcorresponding to the light guide thickness t) in the light emitting face33 in an X-direction crossing the light incident face 31 at rightangles. When a logarithm of the light intensity of the emitted lightfrom the light emitting face 23 is indicated on the ordinate, (L/t) isindicated on the abscissa, and the relation is plotted, the lightemission ratio α can be obtained from gradient.

A lens face on which a large number of elongated lenses extending in adirection (X-direction) substantially vertical to the light incidentface 31 are arranged is preferably formed on the other major surface towhich a directivity light emission function structure is not imparted inorder to control the directivity of the emitted light from the lightguide 3 in a plane (YZ plane) parallel to the primary light source 1. Inthe embodiment shown in FIG. 1, a rough surface is formed on the lightemitting face 33, and a lens face constituted of arrangement of a largenumber of elongated lenses extending in a direction (X-direction)substantially vertical to the light incident face 31 is formed on theback surface 34. In the present invention, conversely to the mode shownin FIG. 1, the lens face may be formed on the light emitting face 33,and the back surface 34 may be formed into a rough surface.

When the elongated lenses are formed on the back surface 34 or lightemitting face 33 of the light guide 3 as shown in FIG. 1, examples ofthe elongated lenses include elongated prisms, elongated lenticularlenses, V-shaped grooves and the like extending substantially in theX-direction. Elongated prisms in which a shape of a YZ section issubstantially triangular are preferable.

In the present invention, when the elongated prisms are formed as theelongated lenses formed on the light guide 3, an apex angle thereof ispreferably set to a range of 70 to 150°. When the apex angle is set tothis range, the emitted light from the light guide 3 can be sufficientlycondensed, and the luminance of the planar light source device can besufficiently enhanced. That is when the prism apex angle is set to thisrange, it is possible to emit the condensed emitted light whose fullwidth at half maximum of the emitted light distribution is 35 to 65° ina plane including peak light of the emitted light distribution (in theXZ plane) and extending vertically to the XZ plane, and the luminance ofthe planar light source device can be enhanced. It is to be noted thatwhen the elongated prisms are formed on the light emitting face 33, theapex angle is preferably set to a range of 80 to 100°. When theelongated prisms are formed on the back surface 34, the apex angle ispreferably set to a range of 70 to 80° or 100 to 150°.

It is to be noted that in the present invention, instead of or inaddition to the above-described light emission function structure formedon the light emitting face 33 or the back surface 34, light diffusingarticulates may be mixed/dispersed in the light guide to thereby i thedirective light emission function. The light guide 3 is not limited tothe sectional shape shown in FIG. 1, and various sectional shapes suchas a wedge shape and a boat shape are usable.

A polarization component reflected by the polarization separationelement 6 enters the backlight unit again, is reflected by the backsurface of the Light guide 3, and enters the polarization separationelement 6 again. At this time, it is preferable that polarized stateshave been changed and a component which passes through the polarizationseparation element 6 has increased. In this case, the direction in whichthe elongated lenses formed on the light guide 3 extends preferablyobliquely crosses a direction of a transmission polarization face of thepolarization separation element 6. In this constitution, the polarizedstate is easily changed by the elongated lenses of the light guide 3.

A linear light source extending in a Y-direction is usable as theprimary light source 1. For example, a fluorescence lamp or a coldcathode tube is usable as the primary light source 1. It is to be notedthat in the present invention, the primary light source 1 is not limitedto the linear light source, and spot light sources such as an LED lightsource, halogen lap, and metallic halide lamp are usable. Especially,when the light source is used in a display device having a cooperativelysmall screen dimension, such as a cellular phone and a portableinformation terminal, a small spot light source such as an LED ispreferably used. The primary light source 1 is disposed facing one sideend face of the light guide 3 as shown in FIG. 1, but may be furtherdisposed on the other side end face facing the above one side end face,if necessary.

For example, as shown in FIG. 7, when a substantially spotted lightsource such as an LED light source used as the primary light source 1 isdisposed on a corner or the like of the light guide 3, the light thathas entered the light guide 3 radially propagates therein substantiallycentering on the primary light source 1 in the same plane as the lightemitting face 33. The emitted light which goes out of the light emittingface 33 similarly goes out radially centering on the primary lightsource 1. The elongated prism formed on the light deflection element 4is preferably substantially juxtaposed and arranged substantially insuch an arc shape to surround the primary light source 1 in order todeflect the radially emitted light in a desired direction withsatisfactory efficiency regardless of the emission direction. When theelongated prism is substantially juxtaposed and arranged substantiallyin such an arc shape to surround the primary light source 1 in thismanner, most of the light radially emitted from the light emitting face33 enters the elongated prism of the light deflection element 4substantially vertically. Therefore, the emitted light can be directedin a specific direction with satisfactory efficiency in the whole regionof the light emitting face 33 of the light guide 3, and especially theseparation ability of the polarization separation element 6 can beenhanced as described above. Furthermore, the luminance can also beenhanced. As to the substantially arc-shaped elongated prism formed onthe light deflection element 4, a degree of the arc shape is selected inaccordance with distribution of the light propagating in the light guide3, and most of the light emitted radially from the light emitting face33 preferably substantially vertically enters the elongated prism of thelight deflection element 4. Concretely, for example, the elongatedprisms are substantially juxtaposed and arranged in a concentric shapesubstantially centering on the spot light source like the LED in such amanner that a radius of circular arc gradually increases. A range of theradius of the elongated prism is determined by the position of the spotlight source in the planar light source system, and a positionalrelation or a size with respect to an effective area of the planar lightsource corresponding to a liquid crystal display area.

The light source reflector 2 guides the light of the primary lightsource 1 into the light guide 3 with little loss. For example, a plasticfilm having a metal evaporated reflective layer on the surface thereofis usable as a material. As shown in FIG. 1, the light source reflector2 is wound to a light outgoing surface edge portion of the lightdeflection element 4 from an edge portion outer surface of the lightreflection element via the outer surface of the primary light source 1.On the other hand, the light source reflector 2 may be wound to thelight emitting face edge portion of the light guide 3 from the edgeportion outer surface of the light reflection element 5 via the outersurface of the primary light source 1 avoiding the light deflectionelement 4.

A reflective member similar to the light source reflector 2 may beattached to a side end face other than the side end face 31 of the lightguide 3. For example, the plastic sheet having the metal evaporatedreflective layer on the surface thereof is usable as the lightreflection element 5. In the present invention, a light reflective layeror the like formed on the back surface 34 of the light guide 3 by metalevaporation or the like may be used as the light reflection element 5instead of the reflective sheet.

It is to be noted that the reflective sheet is preferably disposed asthe light reflection element 5 on the back surface of the light guide 3in order to securely reflect the light reflected by the polarizationseparation element 6 and returned on the backlight unit and to allow thelight to enter the polarization separation element 6 again. A sheet-likeregularly reflective member whose surface is coated with a metal, or asheet-like diffusion reflective member formed of a white PET film or thelike is usable as the reflective sheet. The reflective sheet 5 ispreferably provided with a concave/convex shape to thereby promote achange of the polarized state. For example, a reflective sheet on whicha large number of corner cubes are arranged is usable.

The light guide 3 and the light deflection element 4 in the planar lightsource device of the present invention ray comprise a synthetic resinhaving high light transmittance. Examples of the synthetic resin includea methacrylic resin, acrylic resin, polycarbonate-based resin,polyester-based resin, and vinyl chloride based resin. Especially, themethacrylic resin has high light transmittance, and superior heatresistance, mechanical property, and forming/working property, and isoptimum. As the methacrylic resin, a resin mainly composed of methylmethacrylate and containing 80 weight % or more methyl methacrylate ispreferable, When a surface structure of the rough surface, the elongatedprism or the like of the light guide 3 or the light deflection element 4is formed, the structure may be formed by thermally pressing atransparent synthetic resin plate using a die member having a desiredsurface structure, or the shape may be formed simultaneously withmolding by screen printing, extrusion, injection or the like. Thestructure surface may be formed using a thermal or photo setting resinor the like. Furthermore, a rough surface structure or an elongated lensarrangement structure formed of an active energy ray setting resin maybe formed on the surface of a transparent substrate or a transparentfilm or sheet formed of a polyester-based resin, acrylic resin,polycarbonate-based resin, vinyl chloride based resin, polymethacrylicimide based resin or the like, or the sheet may be bonded/integratedonto/with a separate transparent substrate by methods such as bondingand fusion-bonding. As the active energy ray setting resin, amultifunctional (meth)acrylic compound, vinyl compound, (meth)acrylicester, aryl compound, (meth)acrylic metal salt or the like is usable.

The liquid crystal display element 7 is disposed on a light emissionsurface (light outgoing surface of the polarization separation element6) of the planar light source device constituted of the above-describedprimary light source 1, light source reflector 2, light guide 3, lightdeflection element 4, light reflection element 5, and polarizationseparation element 6, and accordingly a liquid crystal display device isconstituted. The liquid crystal display device is observed by anobserver through the liquid crystal display element from above inFIG. 1. In the present invention, since sufficiently collimated lighthaving a narrow luminance distribution can be applied into thepolarization separation element 6, high polarization separation abilityis obtained, and luminance during the observing through the liquidcrystal display element 7 becomes high. Since the sufficientlycollimated light can be applied into the liquid crystal display element7, satisfactory image display having brightness and hue uniformitywithout any gradation reverse is obtained with the liquid crystaldisplay element 7. Moreover, light irradiation concentrated in a desireddirection is obtained, and use efficiency of the emitted light quantityof the primary light source with respect to illumination in thisdirection.

Furthermore, in the present invention, a light diffusion element may bedisposed in order to appropriately control a view field range inaccordance with a purpose without causing any drop of luminance ifpossible in the planar light source device whose view field is narrowedand whose luminance is raised by the light deflection element 4 asdescribed above. When the light diffusion element is disposed in thismanner, glare, luminance unevenness and the like causing a drop ofquality level are suppressed, and the quality level can be enhanced.This will be further described hereinafter.

The light diffusion element may be integrated with the light deflectionelement 4 on the side of the light outgoing surface of the lightdeflection element 4, or may be individually disposed, and it ispreferable to individually dispose the light diffusion element. Forexample, the light diffusion element may take the form of a lightdiffusing sheet. The light diffusion element is preferably disposed onthe light emitting surface of the polarization separation element 6 inorder to enhance the separation ability by the polarization separationelement 6. By this arrangement, after the light condensed in thenormal-line direction enters the polarization separation element 6, andis polarized/separated, view field range adjustment or quality levelenhancement is performed by the light diffusion element. Aconcave/convex structure is preferably formed on the surface (incidentsurface) of the light diffusion element adjacent to the polarizationseparation element 6 in order to prevent sticking to the polarizationseparation element 6. Similarly, prevention of sticking of the lightdiffusion element to the display element 7 disposed on the lightemitting surface of the light diffusion element needs to be considered,and it is preferable to form the concave/convex structure on the lightemitting surface of the light diffusion element. When thisconcave/convex structure is applied only for a purpose of preventing thesticking, the structure has an average inclination angle of preferably0.7° or more, further preferably 1° or more, more preferably 1.5° ormore. Even when the light diffusion element is disposed on the side ofthe light emitting face of the liquid crystal display element 7, thesimilar effect is obtained.

In the present invention, it is preferable to use the light diffusionelement having a light diffusion characteristic for appropriatelydiffusing the emitted light from the light deflection element 4 inconsideration of balances of luminance characteristic, visibility,quality level and the like. That is, when the light diffusion propertyof the light diffusion element is low, it is difficult to sufficientlybroaden a view field angle, and there is a tendency that visibilityimproving effect or quality level improving effect is not sufficient.Conversely, when a light diffusing property is excessively high, aneffect of narrowing the view field by the light deflection element 4 isimpaired, total light ray transmittance also drops, and there is atendency that the luminance drops. Therefore, in the present invention,an element whose full width at half maximum of the emitted lightdistribution (in the XZ plane) when applying parallel light is in arange of 1 to 13° is used as the light diffusion element. The full widthat half maximum of the light diffusion element is in a range ofpreferably 3 to 11°, further preferably 4 to 8.5°. It is to be notedthat in the present invention, the fall width at half maximum of theemitted light distribution (in the XZ plane) of the light diffusionelement indicates a degree by which the parallel light ray applied intoa light diffusion element 8 having a light incident surface 81 and alight emitting surface 82 as shown in FIG. 8 diffuses and spreads whenbeing emitted, and refers to an angle (Δθ_(H)) of the full width of thespread angle at a half value with resect to a peak value in the emittedlight distribution (in the XZ plane) of the light transmitted anddiffused through the light diffusion element B.

The light diffusion characteristic can be imparted, when a lightdiffusing agent is mixed in the light diffusion element 8, or theconcave/convex structure is formed on at least one surface of the lightdiffusion element 8. A degree of the concave/convex structure formed onthe surface differs in a case where the structure is formed on onesurface of the light diffusion element 8 and in another case where thestructures are formed on opposite surfaces. When the concave/convexstructure is formed on one surface of the light diffusion element 8, theaverage inclination angle is set to a range of preferably 0.8 to 12°,further preferably 3.5 to 7°, further preferably 4 to 6.5°. When theconcave/convex structures are formed on the opposite surfaces of thelight diffusion element 8, the average inclination angle of theconcave/convex structure formed on one surface is set to a range ofpreferably 0.8 to 6°, further preferably 2 to 4°, more preferably 2.5 to4°. In this case, the average inclination angle of the light diffusionelement 8 on the light incident surface side is preferably set to belarger than that on the light emitting surface side. It is preferable toset a haze value of the light diffusion element 8 to a range of 8 to 82%from viewpoints of luminance characteristic enhancement and visibilityimprovement, and the value is in a range of further preferably 30 to70%, more preferably 40 to 65%.

In the present invention, according to method B of JIS K-7105, the hazevalue can be calculated and obtained from a total light raytransmittance (Tt) and a diffused light ray transmittance (Td) obtainedusing an integrating sphere system reflection transmittance gauge(RT-100 type manufactured by Murakemi Sikisai Gijutsu Kenkyusha) by thefollowing equation (4) with respect to a sample having a size of 50mm×50 mm:Haze value (%)=100(Td/Tt)  (4)

In the planar light source device of the present invention, it isdemanded that luminance in a display area be uniform in case ofobservation of the light emission surface (e.g., the light emittingsurface of the light diffusion element 8) from the normal-linedirection. The uniformity of the luminance depends also on the size ofthe display area of the planar light source device. For example, in alarge-sized planar light source device having a large display area for anotebook-size personal computer, monitor or the like, a comparativelybroad view field angle characteristic is sometimes demanded, and it isdemanded that the distribution (in the XZ plane) of the emitted lightfrom the light emission surface be broader. On the other hand, in asmall-sized planar light source device having a small display area for acellular phone, portable information terminal or the like, highluminance or display quality level enhancement sometimes has priority,and the distribution (in the XZ plane) of the emitted light from thelight emitting surface may be comparatively small. Therefore, as thelight diffusion element 8, an element having an appropriate lightdiffusing characteristic is preferably used in accordance with a size ofthe display area of the planar light source device.

In the present invention, an element having anisotropy in the lightdiffusion property is preferably used as the light diffusion element, sothat the total light ray transmittance of the light diffusion element isenhanced, the emitted light from the polarization separation element 6can be efficiently diffused, and the luminance can be enhanced. Forexample, in the planar light source device in which a linear coldcathode tube is disposed as the primary light source 1 facing one endface of the light guide 3, in the light deflection element 4 fornarrowing the view field, mainly the view field of the emitted lightfrom the light emitting face of the light guide 3 is narrowed in the XZplane. The light diffusion element has a purpose to diffuse the light ofthe above narrowed view field mainly in the XZ plane and to broaden theview field angle. However, when using the light diffusion element havingan isotropic diffusing property, the light of the YZ plane whose viewfield is not narrowed is equally diffused by the light diffusionelement, and therefore the drop of luminance is caused. Therefore, bythe use of the light diffusion element having such an anisotropicdiffusing property that the light diffusing property in the XZ plane ishigher than that in the YZ plane, the light of the XZ plane whose viewfield is narrowed by the light deflection element 4 is stronglydiffused, the diffusion of the light of the YZ plane whose view field isnot narrowed can be weakened, the emitted light from the lightdeflection element 4 can be efficiently diffused, and the drop of theluminance can be minimized as much as possible.

The present invention will be described hereinafter concretely inaccordance with examples and comparative example.

It is to be noted that measurements of characteristic values in thefollowing examples and comparative example were performed as follows.

Measurement of Normal-Line Luminance and Luminance Full Width at HalfMaximum of Planar Light Source Device

A cold cathode tube was used as a primary light source, DC12V wasapplied to an inverter (HIU-742A manufactured by Harrison Co.) of adriving circuit to thereby light the cold cathode tube at highfrequency. The normal-line luminance was obtained by dividing thesurface of a planar light source device into 3×5 square portions eachhaving four 20 mm sides, and taking an average of 15 luminance values inthe normal-line direction of each square portion. The view field angleof the luminance meter was set to 0.1°, a measurement position wasadjusted into a middle of a light emission surface of the planar lightsource device, and further adjustment was performed in such a manner asto rotate a rotational shaft of a goniometer. While rotating therotational shaft at an interval of 10 from +90° to −90° in eachdirection, the luminance distribution (in the XZ plane, YZ plane) of theemitted light was measured with a luminance meter, and a peak luminance,and a full width at half maximum (spread angle of distribution of valueswhich were ½ or more of the peak value) of luminance distribution (inthe XZ and YZ planes) were obtained.

Measurement of Luminance Rise Ratio by Polarization Separation Element

A planar light source device in which any polarization separationelement was not incorporated was prepared, and a liquid crystal displayelement 7 was disposed on the light emitting face of a planar lightsource device. In this state, an average of 15 luminance values in anormal-line direction was obtained by the above-described method. Next,a polarization separation element 6 was incorporated in the same planarlight source device. At this time, a polarization transmission directionof the liquid crystal display element 7 on a light incident side wasbrought in parallel with a transmission polarization plane of thepolarization separation element 6. In this state, an average of 15luminance values in the normal-line direction was obtained in theabove-described method. A ratio of a measured value in a case where thepolarization separation element 6 is used with respect to that inanother case where the polarization separation element 6 is not used isassumed as a luminance rise ratio by the polarization separation element6.

Measurement of Average Inclination Angle (θa)

Surface roughness of a rough surface was measured at a driving speed of0.03 nanosecond with a probe type surface roughness gauge (Surfcom 570Amanufactured by Tokyo Seiki Co., Ltd.) using 010-2528 (1 μmR, 55° cone,diamond) as a probe according to ISO4287/1-1987. An average line wassubtracted from a chart obtained by the measurement to thereby correctinclination, and the average inclination angle was calculated andobtained by the above equations (1) and (2).

COMPARATIVE EXAMPLE 1

A light guide whose one major surface was a mat-finished surface havingan average inclination angle of 2.5° and whose other major surface was amirror surface was prepared by injection molding using an acrylic resin(Acripet VH5#000 manufactured by Mitsubishi Rayon Co., Ltd.). The lightguide had a wedge plate shape having a size of 216 mm×290 mm and havinga thickness changing to 2.0 mm–0.7 mm along the side (short side) havinga length of 216 mm. A prism layer was formed of an acrylic ultravioletsetting resin in which elongated prisms each having a prism apex angleof 100° were juxtaposed and arranged at a pitch of 50 μm in parallelwith the short side of the light guide on the mirror surface side of thelight guide. A cold cathode tube was disposed along one side end face(end face on the side having a thickness of 2.0 mm) corresponding to aside (long side) of the light guide having a length of 290 mm. The coldcathode tube was covered with a light source reflector (silverreflective film manufactured by Reikosha Co.). Furthermore, a lightdiffusion reflective film (E60 manufactured by Toray Industries, Inc.)was attached to the other side end face, and a reflective sheet (E60manufactured by Toray Industries, Inc.) was disposed on the surface(back surface) of the elongated prism arrangement. The above-describedconstitution was incorporated in a frame member.

On the other hand, a prism sheet (light deflection element 4) in whichan elongated prism formed surface was formed on one surface of apolyester film having a thickness of 125 μm was prepared using anacrylic ultraviolet ray setting resin having a refractive index of1.5064. On the elongated prism formed surface, elongated prisms weresubstantially juxtaposed and arranged at a pitch of 56.5 μm, first andsecond prism faces constituting each elongated prism were both flatfaces, and each angle (α, β) formed by the prism face with a normal linewas 32.5°.

The obtained prism sheet was disposed on the light guide in such amanner that the elongated prism formed surface was directed on a lightemitting face side of the light guide, a prism ridgeline was parallel toa light incident face of the light guide, and the first prism face wasdisposed on a primary light source side, to thereby obtain the planarlight source device.

In the planar light source device, an emitted light luminancedistribution (in an XZ plane) in a plane vertical to both the lightincident face and the light emitting face, and an emitted lightluminance distribution (in a YZ plane) in a plane parallel to the lightincident face and vertical to the light emitting face were obtained, andan angle (full width at half maximum) having a luminance being ½ of apeak luminance was measured. Results are shown in Table 1.

A liquid crystal display element was disposed on the light emissionsurface of the planar light source device. At this time, a transmissionaxis of a polarization plate on an incident side of the liquid crystaldisplay element was inclined by 45° with respect to the light incidentface of the light guide. In this state, the whole surface of the liquidcrystal display element was brought into a uniform display state, 15values of normal-line luminance were measured by the above-describedmethod, and averaged, to thereby obtain a luminance value A. On theother hand, a plurality of sheets each having a double refractionproperty and a predetermined thickness were stacked, while adjustingdirectionality of each sheet in such a manner that a refractive indexdifference between the adjacent sheets was large in a transmissionpolarization plane and small in a reflection polarization plane, tothereby prepared a polarization separation element. The polarizationseparation element was disposed between the light deflection element 4and the liquid crystal display element 7 of the above-described planarlight source device in such a manner that a transmission axis wasparallel to that of a polarization plate on the incident side of theliquid crystal display element. In this state, the whole surface of theliquid crystal display element was uniformly brought into the samedisplay state as that in a case where the luminance value A wasobtained, 15 values of the normal-line luminance were measured andaveraged in the above-described method, to thereby obtain a luminancevalue B. A luminance rise ratio B/A by the polarization separationelement was calculated. Results are shown in Table 1.

EXAMPLE 1

A planar light source device was prepared in the same method as that ofComparative Example 1 except that the following was used as a lightdeflection element 4.

A prism sheet in which an elongated prism formed surface was formed onone surface of a polyester film having a thickness of 125 μm wasprepared using an acrylic ultraviolet ray setting resin having arefractive index of 1.5064. On the elongated prism formed surface,elongated prisms were substantially juxtaposed and arranged at a pitchof 56.5 μm. One prism face (first prism face) constituting the elongatedprism was a flat face having an angle (α) of 32.5° formed with a normalline. The other prism face (second prism face) comprised two convexcurved faces: a convex curved face (inclination angle=56.60, β=33.8°)extending through an area of elongated prism height of 0 (a prism apexportion) to 21.4 μm from a prism apex portion and forming a part(vicinity of a portion having a curvature radius of 800 μm) of anon-circular arc sectional shape having a curvature radius of 400 μm ona long diameter and a curvature radius of 800 μm on a short diameter;and another convex curved face (inclination angle=59.0°) extendingthrough another area of elongated prism height of 21.4 μm or more fromthe prism apex portion and forming a circular sectional shape having acurvature radius of 400 μm. A ratio (d/P) of a maximum distance (d) froma virtual flat plane of the second prism face of the prism sheet withrespect to a pitch (P) of the elongated prisms was 1.03%.

As to the obtained planar light source device, the full width at halfmaxim of an emitted light luminance distribution in XZ and YZ planes,and a luminance rise ratio by the polarization separation element weremeasured in the same manner as in Comparative Example 1. Results areshown in Table 1.

EXAMPLE 2

A planar light source device was prepared in the same method as that ofComparative Example 1 except that the following was used as a lightdeflection element 4.

A prism sheet was prepared in the sane manner as in Example 1 exceptthat a second prism face constituting the elongated prism comprisedseven flat faces: a flat face (β=34.8°) having an inclination angle of55.2° and extending through an area of elongated prism height of 0 (aprism apex portion) to 16 μm from a prism apex portion; and six flatfaces extending through another area of the elongated prism height of 16μm or more from a prism apex portion and each having inclination anglesof 55.5°, 56.2°, 57.0°, 57.8°, 58.4° and 59.4°, respectively, form aside close to the prism apex portion and an equal width. A ratio (d/P)of a maximum distance (d) from a virtual flat plane of the second prismface of the prism sheet with respect to a pitch (P) of the elongatedprisms was 1.10%.

As to the obtained planar light source device, the full width at halfmaximum of an emitted light luminance distribution in XZ and YZ planes,and a luminance rise ratio by the polarization separation element weremeasured in the same manner as in Comparative Example 1. Results areshown in Table 1.

EXAMPLE 3

A planar light source device was prepared in the same method as that ofComparative Example 1 except that the following was used as a lightdeflection element 4.

A prism sheet was prepared in the same manner as in Example 1 exceptthat a second prism face constituting the elongated prism comprised twoflat faces and one convex curved face: a flat face (β=33.6) having aninclination angle of 56.4° and extending through an area of elongatedprism height of 0 (a prism apex portion) to 10.6 μm from a prism apexportion; another flat face having an inclination angle of 56.8° andextending through another area of elongated prism height of 10.6 to 21.3μm from a prism apex portion; and a convex curved face (inclinationangle 59.2°) of a circular arc sectional shape having a curvature radiusof 400 μm extending through still another area of elongated prism heightof 21.3 μm or more from the prism apex portion. A ratio (d/P) of amaximum distance (d) from a virtual flat plane of the second prism faceof the prism sheet with respect to a pitch (P) of the elongated prismswas 1.03%.

As to the obtained planar light source device, the full width at halfmaximum of an emitted light luminance distribution in XZ and YZ planes,and a luminance rise ratio by the polarization separation element weremeasured in the same manner as in Comparative Example 1. Results areshown in Table 1.

EXAMPLE 4

A planar light source device was prepared in the same method as that ofComparative Example 1 except that the following was used as a lightdeflection element 4, and an elongated prism pitch thereof was set to 50μm.

A prism sheet was prepared in the same manner as in Example 1 exceptthat each of first and second prism faces constituting the elongatedprism comprised one convex curved face (α=32.7°, β=32.7°, inclinationangle=57.3°) of a circular arc sectional shape having a curvature radiusof 250 μm. A ratio (d/P) of a maximum distance (d) from a virtual flatplane of the second prism face of the prism sheet with respect to apitch (P) of the elongated prisms was 2.14%.

As to the obtained planar light source device, the full width at halfmaximum of an emitted light luminance distribution in XZ and YZ planes,and a luminance rise ratio by the polarization separation element weremeasured in the same manner as in Comparative Example 1. Results areshown in Table 1.

EXAMPLE 5

A planar light source device was prepared in the same method as that ofComparative Example 1 except that the following was used as a lightdeflection element 4, and an elongated prism pitch thereof was set to 50μm.

A prism sheet was prepared in the same manner as in Example 1 exceptthat each of first and second prism faces constituting the elongatedprism comprised one convex curved face (α=32.7°, β=32.7°, inclinationangle=57.3°) of a circular arc sectional shape having a curvature radiusof 425 μm. A ratio (d/P) of a maximum distance (d) from a virtual flatplane of the second prism face of the prism sheet with respect to apitch (P) of the elongated prisms was 1.26%.

As to the obtained planar light source device, the full width at halfmaximum of an emitted light luminance distribution in XZ and YZ planes,and a luminance rise ratio by the polarization separation element weremeasured in the same manner as in Comparative Example 1. Results areshown in Table 1.

EXAMPLE 6

A light guide was prepared by injection molding using an acrylic resin(Acripet VH5#000 manufactured by Mitsubishi Rayon Co., Ltd.). The lightguide had a wedge plate shape having a size of 40 mm×50 mm and having athickness changing to 0.8 mm–0.6 mm along the side (long side) having alength of 50 mm. A corner portion positioned between one side end face(end face on the side having a thickness of 0.8 mm) corresponding to theside (short side) of the light guide having a length of 40 mm and thelong side of the light guide was cut, to thereby form a light incidentface. A rough surface having an average inclination angle of 2.5° wasformed on the light emitting face of the light guide. An LED (NSCW215Rmanufactured by Nidia Kagaku Co.) was disposed in close contact with thelight incident face. Furthermore, a light diffusion reflective film (E60manufactured by Toray Industries, Inc.) was attached to the other sideend faces, and a reflective sheet (E60 manufactured by Toray Industries,Inc.) was disposed on the surface (back surface) of the elongated prismarrangement. The above-described constitution was incorporated in aframe member.

On the other hand, a prism sheet in which elongated prisms were formedon one surface of a polyester film having a thickness of 125 μm wasprepared using an acrylic ultraviolet ray setting resin having arefractive index of 1.5064. Elongated prisms had the same shapes offirst and second prism faces as those of Example 2 and a pitch of 56.5μm and were formed into concentric shapes centering on one corner of theprism sheet.

The obtained prism sheet was disposed in such a manner that a center ofa concentric circular arc of the formed elongated prisms fell on acenter of a light emission face of the primary light source, and theelongated prism formed surface was directed on a light emitting faceside of the light guide, to thereby obtain the planar light sourcedevice.

As to the obtained planar light source device, the full width at halfmaximum of an emitted light luminance distribution in XZ and YZ planes,and a luminance rise ratio by a polarization separation element weremeasured in the same manner as in Comparative Example 1. Results areshown in Table 1. Additionally, during measurement of the luminance riseratio, a normal-line luminance was measured in a state in which atransmission axis of a polarization plate on a light incident side of aliquid crystal display element was vertical to the light guide lightemitting face, to thereby obtain a luminance value A. A normal-lineluminance was measured in a state in which the polarization separationelement was disposed between a light deflection element and liquidcrystal display element of the planar light source device in such amanner that a transmission axis was parallel to that of the polarizationplate on the light incident side of the liquid crystal display element,to thereby obtain a luminance value B.

TABLE 1 XZ plane luminance YZ plane luminance full width at half fullwidth at half Luminance rise maximum maximum ratio (B/A) Comparative27.5° 40.2° 1.38 Example 1 Example 1 14.1° 40.2° 1.46 Example 2 14.5°40.2° 1.44 Example 3 13.9° 40.2° 1.47 Example 4 21.2° 40.2° 1.42 Example5 15.3° 40.2° 1.45 Example 6 14.5° 16.4° 1.55

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, a distributionis constituted in such a manner that light entering a polarizationseparation element is condensed in a normal-line direction of thepolarization separation element, accordingly a polarization separationability of the polarization separation element is enhanced, and therecan be provided a planar light source device having a remarkably highluminance based on a specific polarization component. Such a planarlight source device is used in combination with a display device usinglight of the specific polarization component, such as a liquid crystaldisplay device, so that display having a remarkably high luminance atlow power consumption can be realized.

1. A planar light source device comprising: a primary light source; alight guide leading light emitted from the primary light source, andhaving a light incident face to which the light emitted from the primarylight source comes in, and a light emitting face from which the lightgoes out; a light deflection element disposed adjacent to the lightemitting face of the light guide; and a polarization separation elementdisposed on the side of a light outgoing surface of the light deflectionelement, and having a function of transmitting one of polarizationcomponents of an incident light and reflecting the other of thepolarization components, wherein a full width at half maximum of aluminance angular distribution of light incident upon the polarizationseparation element in a direction parallel to a travel direction of thelight in the light guide is 25° or less.
 2. The planar light sourcedevice according to claim 1, wherein the full width at half maximum ofthe luminance angular distribution of light incident upon thepolarization separation element in a direction vertical to the traveldirection of the light in the light guide is 50° or less.
 3. The planarlight source device according to claim 1, wherein an average value ofthe full width at half maximum of the luminance annular distribution oflight incident upon the polarization separation element in directionsvertical and parallel to the travel direction of the light in the lightguide is 33° or less.
 4. The planar light source device according toclaim 1, wherein the light deflection element has a light incomingsurface positioned facing the light emitting face of the light guide anda light outgoing surface on the opposite side, and a plurality ofelongated prisms extending substantially in parallel with one anotherare formed at least on the light incoming surface.
 5. The planar lightsource device according to claim 1, wherein the primary light sourcecomprises a spotlight source, the light deflection element has a lightincoming surface positioned facing the light emitting face of the lightguide and a light outgoing surface on the opposite side, and a pluralityof substantially arc-shape elongated prisms surrounding the primarylight source are juxtaposed and formed at least on the light incomingsurface.
 6. The planar light source device according to claim 4, whereineach of the elongated prisms of the light deflection element comprisestwo prism faces, and at least one of the prism faces is a face otherthan a single flat face.
 7. The planar light source device according toclaim 6, wherein at least one of the prism faces includes at least oneconvex curved face.
 8. The planar light source device according to claim7, wherein at least one of the prism faces comprises at least one convexcurved face, and at least one flat face, an inclination angle of theconvex curved face or the flat face positioned on the side closer to thelight outgoing surface is larger, and a difference between theinclination angle of the convex curved face or the flat face closest tothe light outgoing surface and that of the flat face or the convexcurved face most distant from the light outgoing surface is 15° or less.9. The planar light source device according to claim 7, wherein at leastone of the prism faces comprises at least two convex curved faces havingmutually different inclination angles, an inclination angle of theconvex curved face positioned on the side closer to the light outgoingsurface is larger, and a difference between the inclination angle of theconvex curved face closest to the light outgoing surface and that of theconvex curved face most distant from the light outgoing surface is 15°or less.
 10. The planar light source device according to claim 6,wherein at least one of the prism faces comprises at least two flatfaces having mutually different inclination angles, an inclination angleof any one of the flat faces positioned on the side closer to the lightoutgoing surface is larger, and a difference between the inclinationangle of a flat face closest to the light outgoing surface and that ofthe flat face most distant from the light outgoing surface is 15° orless.
 11. The planar light source device according to claim 7, wherein aratio (r/P) of a curvature radius (r) of the convex curved face to apitch (P) of the elongated prism is in a range of 2 to
 50. 12. Theplanar light source device according to claim 8, wherein at least twoflat faces and/or convex curved faces are formed in a region having aheight h from a prism apex portion, and h/H is 60% or less assuming thatthe height of the elongated prism is H.
 13. The planar light sourcedevice according to claim 8, wherein a ratio of a maximum distance (d)between the flat face and/or the convex curved face, and a virtual planeconnecting a prism apex portion to a prism bottom portion with respectto the pitch (P) of the elongated prism is in a range of 0.05 to 5%. 14.The planar light source device according to claim 4, wherein an apexangle of the elongated prism is in a range of 35 to 80°.
 15. The planarlight source device according to claim 4, wherein one distributing angleα of an apex angle of the elongated prism is 40° or less, and the otherdistributing angle β of the apex angle is in a range of 25 to 50°. 16.The planar light source device according to claim 4, wherein onedistributing angle α of an apex angle of the elongated prism isdifferent from the other distributing angle β.
 17. The planar lightsource device according to claim 4, wherein each of the elongated prismsof the light deflection element comprises two prism faces, one of theprism faces comprises a flat face and/or a convex curved face, and theother prism face is a substantially flat face.
 18. The planar lightsource device according to claim 1, wherein a plurality of elongatedlenses extending in a direction substantially vertical to the lightincident face of the light guide and arranged substantially in parallelwith one another in a plane along the light emitting face are formed onone of the light emitting face of the light guide and a back surface onthe opposite side thereof.
 19. The planar light source device accordingto claim 1, wherein the polarization separation element comprises aplurality of sheets each having double refractive properties, and arefractive index difference between the adjacent sheets in apolarization direction of a reflected polarization component is smallerthan that in a polarization direction of a transmitted polarizationcomponent.
 20. The planar light source device according to claim 1,further comprising: a light diffusion element disposed on the side of alight emitting surface of the polarization separation element.
 21. Theplanar light source device according to claim 20, wherein the lightdiffusion element has a full width at half maximum of an emitted lightluminous angular distribution with incidence of parallel light in arange of 1 to 13°.
 22. The planar light source device according to claim20, wherein a haze value of the light diffusion element is in a range of8 to 82%.